The present invention relates to an adhesive bonding process for incorporating aluminum alloy bodies into vehicle structures. More specifically, the invention relates to an adhesive bonding process wherein a surface portion of an aluminum alloy body is anodized in an acidic solution in preparation for adhesive bonding to an adjacent metal structure.
Aluminum alloy components have achieved widespread use in aircraft and automotive applications because they are light and have high strength. Aircraft manufacturers join aluminum alloy bodies permanently to other metal structures, for example in stringers, wings, and aluminum-polymer laminates.
Before aluminum alloy bodies are adhesively bonded to other structures they are generally surface treated, for example by anodizing in an acidic solution. Anodizing in phosphoric acid is the preferred surface treatment in North America. Some patents disclosing phosphoric acid anodizing for aerospace structures include Marceu et al U.S. Pat. Nos. 4,085,012 and 4,127,451.
The choice of phosphoric acid over other electrolytes suitable for anodizing is related to the slower hydration rate of the phosphated oxide formed, which makes this oxide layer environmentally stable. Phosphoric acid anodizing produces excellent bond strengths and bond durability, but the process requires about 20 minutes thereby limiting it to batch processing. Accordingly, there still remains a need for a faster anodizing step in order to provide a continuous anodizing and coating process having reduced cost to airframers.
A principal objective of the present invention is to provide an adhesive bonding process including a hyposphosphorous acid anodizing step.
A related objective of the invention is to provide an adhesive bonding process including an acid anodizing step carried out more quickly than conventional, phosphoric acid anodizing processes.
An advantage of the adhesive bonding process of the present invention is that adhesive bond strength and durability are comparable to conventional processes even though processing time is reduced substantially.
Another advantage of the invention is that hypophosphorous acid anodizing avoids the potential environmental problems associated with anodizing processes utilitizing chromium compounds.
Additional objectives and advantages of the present invention will become apparent to persons skilled in the art from the following detailed description of a particularly preferred embodiment.
In accordance with the present invention, there is provided an improved process for adhesively bonding an aluminum alloy body to an adjacent metal structure. As used herein, the term xe2x80x9caluminum alloyxe2x80x9d refers to an alloy containing at least about 85 wt % aluminum and one or more alloying elements that are not subversive to acid anodizing pretreatments. Some suitable alloying elements include copper, manganese, magnesium, silicon, zinc, and lithium. These alloying elements are sometimes called character imparting because alloys containing them derive characteristic properties from such elements.
Usually the amounts of such alloying elements are, as to each of copper, magnesium, and zinc, about 0.5 to 10% by weight of the total alloy; as to the element manganese, usually about 0.15 to 2% of the total alloy; as to silicon, usually about 0.25 to 15% of the total alloy; and as to the element lithium, about 0.2 to 3% of the total alloy. Iron and beryllium may also be present in aluminum alloys and can have a marked effect upon alloys containing them. Iron, for example, is often adjusted in amounts of about 0.3 to 2% by weight to perform specific functions and beryllium may be present in amounts of about 0.001 to 5% of the total alloy.
Various aluminum alloys in the form of sheet, plate, castings, and extrusions are suitable for practice of the present invention. Aluminum alloy sheet and plate are preferred. Suitable alloys for aluminum sheet and plate include heat treatable aluminum-copper alloys of the AA2000 series, aluminum-magnesium-silicon alloys of the AA6000 series, and aluminum-zinc alloys of the AA7000 series.
Aluminum alloy castings suitable for practice of the invention include die castings, sand castings, and permanent mold castings. One suitable casting alloy is alloy A356 with a nominal composition of 7.0 wt % Si, 0.3 wt % Mg, 0.17 wt % max Fe, 0.17 wt % max Cu, remainder aluminum, incidental elements, and impurities. The A356 alloy castings are commonly solution heat treated or aged at an elevated temperature before use. Two other useful alloys are C119 containing 9-10.5 wt % Si and 0.10-0.20 wt % Mg; and C448 containing 9.0-11.5 wt % Si, 0.4-0.8 wt % Mn, and 0.10-0.35 wt % Mg.
Aluminum alloy extrusions suitable for practice of the invention are preferably made from alloys containing silicon and magnesium in proportions allowing for heat treatment, such as the AA6000 series aluminum alloys. The AA6009, 6010, 6061, 6063, and similar alloys are useful, and the AA6061 and 6063 aluminum alloys are particularly preferred. Other useful alloys for extrusions include C210 containing 0.40-0.60 wt % Si, 0.15-0.25 wt % Cu, 0.40-0.60 wt % Mg, and 0.15-0.25 wt % Fe; and C461 containing 0.4-0.6 wt % Si, 0.15-0.40 wt % Fe, 0.45-0.70 wt % Mg, and 0.10-0.25 wt % V. Extruded aluminum alloy shapes are generally solution heat treated and quenched after they leave the extrusion die.
Vehicle assemblies for the aircraft and automotive industries must be built to exacting standards because of the extreme conditions they are subjected to in use. For example, aircraft wing structures for passenger, cargo, and military aircraft utilize adhesively bonded assemblies. Conditions of use involve extreme temperature variations ranging from below zero temperatures in arctic areas and temperatures greater than 150xc2x0 F. (66xc2x0 C.) in tropical areas where the assemblies are exposed to the sun. Aircraft assemblies are also exposed to atmospheric pollutants and other corrosive atmospheres. Of particular importance is resistance to corrosion and delamination of adhesively bonded structures in warm, humid environments. When adhesively bonded, metal-to metal vehicle assmeblies fail they must be replaced, thereby necessitating field repairs or even removal from operating service for extensive times so that repairs may be completed.
The present invention involves forming an anodic oxide coating on at least one surface portion of an aluminum alloy body, thereby rendering the surface portion susceptible to adhesion with various thermosetting or thermoplastic resins. Suitable adhesives include epoxies, polyurethanes, and acrylics. Epoxy adhesives are partiucalarly preferred.
Optionally, the surface portion to be anodized is cleaned to remove lubricants and other contaminants. Suitable surface cleaners include alkaline solutions, acidic solutions, and organic solvents including methylethyl ketone, acetone, and isopropanol.
The cleaned aluminum alloy body is anodized in an electrolyte in order to provide an anodic oxide coating on one or more surface portions, thereby rendering such surface portions suitable for adhesive bonding. The electroylyte contains about 1-25 wt % hypophosphorous acid (H3PO2), preferably about 3-15 wt %. Anodizing voltage is about 5-40 volts, preferably about 5-25 volts. Current density is about 1-50 mA/cm2. An advantage of the invention is that a satisfactory anodic oxide coating can be obtain by anodizing for less than about 1 minute, preferably less than about 45 seconds. A preferred anodizing time is about 10-30 seconds, and a particularly preferred time is about 20 seconds.
Optionally, a suitable primer may be applied to the anodized surface portion before the adhesive. Some suitable primers are phenolic resins, epoxy resins, and epoxy-modified phenolics. A particularly preferred primer is an epoxy-modified phenolic sold by Cytec Industries under the name BR127.
A suitable adhesive is applied to the anodized aluminum alloy surface portion. The adhesive may be a thermoplastic or thermoselting resin selected from the epoxies, polyurethanes, and acrylics. A preferred adhesive is 3 M""s AF-163-2k epoxy adhesive.
At least one layer of adhesive is applied to the anodized surface portion,and the adhesive coated surface portion is bonded to another metal structure to form a vehicle assembly. The other metal structure may comprise a metal selected from aluminum alloys, steel, and titanium.